In multispeed gear transmissions using, for example, a plurality of planetary gear sets whose members must be selectively connected together, braked or connected to various shafts, it is a common practice to provide friction clutches or brakes to effect the interconnection. Such transmissions are frequently operated under load, i.e. neither the load on the output side of the transmission nor the driving torque at the input side of the transmission is decoupled from the gearing during shifting.
Such friction clutches or brakes, hereinafter referred to as friction devices, are generally operated with rising pressure which can result in a sharp torque increase during gear change, especially if the coefficient of friction of the device has a rising gradient or characteristic.
A conventional control arrangement for operating such friction devices is disclosed, for example, in German Pat. No. 25 12 778. This system likewise hydraulically energizes the clutch or brake with a rising pressure although it provides controlled or time-delayed pressurization.
In the device of this patent, a spool-type valve is provided which has, upon the valve slider, a pair of spools adapted respectively to control the connection of a pressurization passage and a venting passage with a passage communicating with the clutch.
This slider is acted upon by clutch pressure against a spring force and is associated with a damping piston movably mounted in the housing of the valve and defining an expandable chamber which communicates with the clutch via a rotor and a check valve.
The diameter of the samping piston, which acts against the slider spring, is greater than the diameter of the slider affected by the clutch pressure so that ultimately the pressure buildup in the expandable chamber will result in an engagement by the damping piston with the spool and a full displacement of the latter into the open position allowing full pressurization of the clutch.
The device can also include a piston, generally a differential piston, displaced by spring pressure toward the spool and biased by the output of a control valve away from the spool so that the pressure spring can urge the spool toward this differential piston thereby communicating the pressure port of the valve with the clutch or brake and blocking the venting port.
In operation, when the differential piston is shifted by the control pressure, the spool is shifted by the spring force and the fluid is permitted to flow from the pressure port to the clutch. The pressure buildup in the clutch, however, is retarded because hydraulic fluid is simultaneously bled through the throttle into the expandable compartment to displace the damping piston which is shifted against the force of the spring toward the spool.
Once the damping piston engages the spool, however, it entrains the latter into its fully open position and a sharp pressure buildup can occur in the clutch or brake. The check valve permits reverse flow of the fluid from the expandable chamber when the clutch or brake is deenergized.
With this system a retarded pressure buildup in the clutch or brake can be ensured but the pressure buildup nevertheless is characterized by a rising pressure.